1. Field of the Invention
The present invention relates to a distortion compensation apparatus, and more particularly a distortion compensation apparatus which obtains a differential signal between a reference signal, that is, a transmission signal and a feedback signal, calculates a distortion compensation coefficient so as to reduce the differential signal by use of an adaptive algorithm, updates a distortion compensation coefficient having been stored using the above-calculated distortion compensation coefficient, and performs distortion compensation onto the transmission signal based on the above obtained distortion compensation coefficient.
2. Description of the Related Art
In recent years, high-efficient digital transmission has been adopted in radio communication. When multilevel phase modulation is adopted in the radio communication, it is an important technique to suppress nonlinear distortion by linearizing the amplification characteristic of a power amplifier on the transmission side, thereby reducing adjacent channel leak power.
Also, when it is intended to improve power efficiency using an amplifier having a degraded linearity, a technique for compensating nonlinear distortion caused by the degraded linearity is essentially required.
FIG. 1 shows an exemplary block diagram of transmission equipment in the conventional radio equipment. A transmission signal generator 1 outputs a digital serial data sequence. A serial-to-parallel (S/P) converter 2 then converts the digital data sequence into two series, in-phase component signals (I-signals) and quadrature component signals (Q-signals), by alternately distributing the digital data sequence on a bit-by-bit basis.
A digital-to-analog (D/A) converter 3 converts the I-signal and the Q-signal into an analog baseband signal, respectively, so as to input into a quadrature modulator 4. This quadrature modulator 4 multiplies the input I-signal and Q-signal (a baseband transmission signal) by a reference carrier wave 8, and a carrier wave phase-shifted by 90° from the reference carrier wave 8, and adds the multiplied results, thus performing orthogonal transformation, and outputs the above signal.
A frequency converter 5 mixes the quadrature modulation signal with a local oscillation signal, and converts the mixed signal into a radio frequency. A transmission power amplifier 6 performs power amplification of the radio frequency signal being output from frequency converter 5, and radiates to the air from an antenna 7.
Here, in the mobile communication using W-CDMA, etc., transmission equipment power is substantially large, as much as 10 mW to several tens of mW, and the input/output characteristic (having a distortion function f(p)) of transmission power amplifier 6 shows non-linearity, as shown by the dotted line in FIG. 2. This nonlinear characteristic produces a non-linear distortion. As shown by the solid line (b) in FIG. 3, the frequency spectrum in the vicinity of a transmission frequency f0 comes to have a raised sidelobe, shifted from the characteristic shown by the broken line (a) in FIG. 3. This produces a leak to adjacent channels, and adjacent interference. Namely, due to the nonlinear distortion shown in FIG. 2, the leak power of the transmission wave to the adjacent frequency channels becomes large, as shown in FIG. 3.
An ACPR (adjacent channel power ratio) represents the magnitude of leak power, being defined as a ratio of leak power to adjacent channels, which corresponds to a spectrum area in the adjacent channels sandwiched between the lines B and B′ in FIG. 3, to the power in the channel of interest, which corresponds to a spectrum area between the lines A and A′. Such the leak power affects other channels as noise, and degrades the communication quality of the channel of interest. For this reason, a strict regulation has been established.
The leak power is substantially small in a linear region of, for example, a power amplifier (refer to a linear region I in FIG. 2), and is substantially large in a nonlinear region II. Accordingly, in order to obtain a high-output transmission power amplifier, the linear region I has to be widened. However, this requires an amplifier having a larger capacity than is actually needed, which causes a disadvantageous problem in both cost and size of the apparatus. To cope with this problem, it has been applied to add a distortion compensation function to radio equipment so as to compensate for the transmission power distortion.
FIG. 4 shows a block diagram of transmission equipment having a digital nonlinear distortion compensation function by use of a DSP (digital signal processor). A digital data group (transmission signals) transmitted from transmission signal generator 1 is converted into two series, I-signals and Q-signals, in S/P converter 2, and then input to a distortion compensator 9 structured of the DSP.
As shown in the lower part of FIG. 4 in enlargement, distortion compensator 9 includes: a distortion compensation coefficient storage 90 storing a distortion compensation coefficient h(pi) corresponding to the power level pi (where i=0-1023) of a transmission signal x(t); a predistortion portion 91 performing a distortion compensation process (predistortion) onto the transmission signal, using the distortion compensation coefficient h(pi) according to the transmission signal power level; and further, a distortion compensation coefficient calculator 92 for updating a distortion compensation coefficient by comparing the transmission signal x(t) with a demodulation signal (a feedback signal) y(t) demodulated in a quadrature detector, which will be described later, and calculating the distortion compensation coefficient h(pi) so that the difference of the above compared values becomes zero.
The signal to which distortion process is performed in distortion compensator 9 is input into D/A converter 3. This D/A converter 3 converts the input I-signal and Q-signal into analog baseband signals, and inputs the converted signals into quadrature modulator 4. Quadrature modulator 4 performs quadrature modulation by multiplying the input I-signal and Q-signal by a reference carrier wave 8 and a carrier wave being phase-shifted by 90° from reference carrier wave 8, respectively. Quadrature modulator 4 performs quadrature modulation by adding the multiplication result, and outputs the modulated signal.
A frequency converter 5 performs frequency conversion by mixing the quadrature modulation signal with a local oscillation signal. A transmission power amplifier 6 performs power amplification of the radio frequency signal being output from frequency converter 5, and radiates to the air from antenna 7.
A portion of the transmission signal is input to a frequency converter 11 via a directional coupler 10, and input into a quadrature detector 12 after being frequency converted by the above frequency converter 11. Quadrature detector 12 performs quadrature detection by multiplying the input signal by a reference carrier wave, and by a signal being phase-shifted by 90° from the reference carrier wave, respectively. Thus, the baseband I-signal and Q-signal on the transmission side are reproduced, and then input into an analog-to-digital (A/D) converter 13.
A/D converter 13 converts the input I-signal and Q-signal into digital signals, and inputs into distortion compensator 9. Through the adaptive signal processing using an LMS (least-mean-square) algorithm, in distortion compensation coefficient calculator 92 of distortion compensator 9, the pre-compensated transmission signal is compared with the feedback signal being demodulated in quadrature detector 12. Then, distortion compensator 9 calculates the distortion compensation coefficient h(p1) so that the difference of the above comparison values becomes zero, and updates the above-obtained coefficient having been stored in distortion compensation coefficient storage 90. Through the repetition of the calculations above, nonlinear distortion in transmission power amplifier 6 is suppressed, and adjacent channel leak power is reduced.
FIG. 5 shows an explanation diagram when the distortion compensation processing is performed using the adaptive LMS algorithm in distortion compensator 9 shown in FIG. 4.
A symbol 15a is a multiplier for multiplying a transmission signal x(t) by a distortion compensation coefficient hn−1(p). This multiplier corresponds to the predistortion portion 91 shown in FIG. 4. Also, 15b is a transmission power amplifier having a distortion function f(p), and 15c is a feedback system in which feedback the output signal y(t) being output from transmission power amplifier 15b is performed. Also, 15d is a calculator (amplitude-power converter) for calculating a power p (=x2(t)) of the transmission signal x(t), and 15e is a distortion compensation coefficient storage (which corresponds to distortion compensation coefficient storage 90 shown in FIG. 4) for storing the distortion compensation coefficients each corresponding to each power of the transmission signal x(t).
Distortion compensation coefficient storage 15e outputs a distortion compensation coefficient hn−1(p) corresponding to the power p of the transmission signal x(t). Distortion compensation coefficient storage 15e also updates a distortion compensation coefficient hn−1(p) with a distortion compensation coefficient hn(p) obtained through the LMS algorithm.
Further, 15f is a conjugate complex signal output portion, 15g is a subtractor outputting a difference e(t) between the transmission signal x(t) and the feedback demodulation signal y(t), 15h is a multiplier multiplying e(t) by u*(t), 15i is a multiplier multiplying hn−1(p) by y*(t), and 15j is a multiplier multiplying by a step size parameter μ, and 15k is an adder adding hn−1(p) to μe(t)u*(t). Also, 15m, 15n, 15p are delay portions by which a delay time D is added to the input signal. Here, the delay time D denotes the time duration from the time the transmission signal x(t) is input to the time the feedback demodulation signal y(t) is input to subtractor 15g. 
Symbols 15f, 15h-15j constitute a calculation section 16. A signal y(t) is the signal after being distorted. The delay time D being set in delay portions 15m, 15n, 15p is determined so as to satisfy D=D0+D1, where D0 is the delay time in transmission power amplifier 15b, and D1 is the delay time in feedback system 15c. 
When this delay time D cannot be set correctly, the distortion compensation function does not work effectively. Also, the greater the set error in the delay time is produced, the greater the leak power to the adjacent channels caused by the raised sidelobe becomes.
Using the above configuration, the following calculations are performed.hn(p)=hn−1(p)+μe(t)u*(t)e(t)=x(t)−y(t)y(t)=hn−1(p)x(t)f(p)u(t)=x(t)f(p)=hn−1(p)*y(t)p=|x(t)|2Here, x, y, f, h, u, e are complex numbers, and * denotes a conjugate complex number.
Through the above calculation processing, the distortion compensation coefficient h(p) is updated so as to minimize the differential signal e(t) between the transmission signal x(t) and the feedback demodulation signal y(t). Finally, the value converges to an optimal distortion compensation coefficient, so that the distortion of the transmission power amplifier is compensated.
As described above, the principle of the distortion compensation apparatus is that feedback detection of a carrier wave obtained after quadrature modulation of the transmission signal is performed, the amplitudes of the transmission signal and the feedback signal are compared after digital conversion, and a distortion compensation coefficient is updated real time based on the above comparison result. According to this nonlinear distortion compensation system, it is possible to reduce distortion, and leak power as well, even through the operation is performed in a nonlinear region with high output, and also to improve the power load efficiency.
Now, in regard to the above setting of the delay time in the prior application, the applicant of the present invention has proposed one method, which is disclosed in the official gazette of the Japanese Unexamined Patent Publication No. 2001-189685. The method disclosed in the above patent document 1 is outlined below: A correlation value is calculated varying the phases between a transmission signal x(t) and a feedback signal. Based on the maximum value of this correlation, a total delay time produced in a distortion device (transmission power amplifier), a feedback loop, etc. is determined. Then, the determined delay time is set in each delay circuit for timing adjustment in the distortion compensation apparatus.
As such, the distortion compensation operation is performed by the distortion compensation apparatus so as to reduce the difference between the transmission signal and the feedback signal. However, due to incompleteness of the distortion compensation, a noise being output from the transmission power amplifier posterior to the distortion compensation may possibly become larger than the noise being output therefrom prior to the distortion compensation, at the end region of the distortion compensation control bandwidth originally having a small distortion signal.
This signifies an undesirable increase of an unwanted wave transmitted at a frequency apart from the transmission bandwidth.
For example, according to a standard in regard to the unwanted wave specified in the specification TS25.104 issued by the 3GPP (3rd Generation Partnership Project), it is required to reduce an unwanted wave so as to be sufficiently small at the frequency having a distance of a predetermined frequency offset amount from the transmission bandwidth.
FIG. 6 shows a diagram concretely explaining the standard of the unwanted wave, in which a transmission signal spectrum exemplifying a case of four carriers is illustrated. In FIG. 6, a frequency F is indicated on the horizontal axis, and a transmission intensity SS is indicated on the vertical axis. The transmission signal output spectrum is shown in the case four carriers are set.
As shown in this FIG. 6, transmission signals for four carriers are disposed within a transmission bandwidth SSBND1, and at the frequency having a distance of a frequency offset shown as OFFSET away from the end of the transmission bandwidth, it is required that an unwanted wave level shall be reduced for the value C or more from the level in the transmission bandwidth.
In FIG. 6, a spectrum A indicates the spectrum prior to the distortion compensation having the transmission bandwidth SSBND1 of four carriers, while a spectrum B indicates the spectrum posterior to the distortion compensation explained in regard to FIG. 5. The floor level in the end region of the transmission bandwidth SSBND1 of four carriers is decreased. On the other hand, the floor is widened in the spectrum.
However, even in the case the spectrum floor is widened, the unwanted wave is suppressed to a low level at the frequency having a distance of the frequency offset amount OFFSET specified in the standard of the unwanted wave, and thereby the standard is met.
Now, a case of setting the number of carriers less than four is considered in the following. FIG. 7 is a diagram illustrating a transmission signal output spectrum when the number of carriers is two. In the case of two carriers, a transmission signal bandwidth SSBND2 becomes narrower than the transmission signal bandwidth SSBND1 in the case of four carriers shown in FIG. 6. Meanwhile, when distortion compensation is performed on the spectrum A prior to the distortion compensation, undesirably, a floor E of a spectrum B becomes widened, similar to the case of four carriers.
Also, since the transmission signal bandwidth SSBND2 becomes narrower than the transmission signal bandwidth SSBND1 in the case of four carriers, the frequency having a distance of the frequency offset amount OFFSET specified in the standard of the unwanted wave is shifted to the left (nearer to the transmission bandwidth) as compared to FIG. 6. In this case, because of the influence of the widened floor E due to the distortion compensation, the unwanted wave level is increased at the frequency having a distance of the frequency offset amount specified in the standard of the unwanted wave. As a result, the specified condition that an unwanted wave level at the aforementioned frequency shall be reduced for the value C or more from the level in the transmission bandwidth is not met any more. Namely, the unwanted wave level has a value D, which is smaller than C, below the transmission bandwidth level.
As such, when the number of carriers (transmission bandwidth) is variable, the distortion compensation may produce an increased unwanted wave (inconformity to the standard of the unwanted wave), contrarily.